php backend for streaming file upload with XMLHttpRequest

Streamer.php

<?php
class File_Streamer
{
	private $_fileName;
	private $_contentLength;
	private $_destination;

	public function __construct()
	{
		if (!isset($_SERVER['HTTP_X_FILE_NAME'])
			&& !isset($_SERVER['CONTENT_LENGTH'])) {
			throw new Exception("No headers found!");
		}

		$this->_fileName = $_SERVER['HTTP_X_FILE_NAME'];
		$this->_contentLength = $_SERVER['CONTENT_LENGTH'];
	}

    public function isValid()
    {
        if (($this->_contentLength > 0)) {
            return true;
        }

        return false;
    }

    public function setDestination($destination)
    {
    	$this->_destination = $destination;
    }

    public function receive()
    {
        if (!$this->isValid()) {
            throw new Exception('No file uploaded!');
        }

        $fileReader = fopen('php://input', "r");
        $fileWriter = fopen($this->_destination . $this->_fileName, "w+");

        while(true) {
            $buffer = fgets($fileReader, 4096);
            if (strlen($buffer) == 0) {
                fclose($fileReader);
                fclose($fileWriter);
                return true;
            }

            fwrite($fileWriter, $buffer);
        }

        return false;
    }

}

upload.php

<?php
require_once('Streamer.php');

$ft = new File_Streamer();
$ft->setDestination('data/');
$ft->receive();